You can add meta data, embedded between the <head> tags in the html of your blog post, to tell Facebook exactly what information to show when that content is shared.
Here are the main meta tags Facebook uses:
<!-- Facebook Open Graph meta data -->
<meta property="og:title" content="Title Here" />
<meta property="og:type" content="article" />
<meta property="og:url" content="http://www.example.com/" />
<meta property="og:image" content="http://example.com/image.jpg" />
<meta property="og:description" content="Description Here" />
You’d simply add the link to the image from the blog post to the <meta property="og:image" content="http://example.com/image.jpg" /> tag to tell Facebook which image to use.
Though as you are most likely not going to be editing the html code for each page yourself, you can use a plugin to add this information instead. I can recommend Yoast SEO, which also gives options for optimising a whole host of SEO settings for your site, however if you just want to add Facebook meta data you could try something like this plugin – though I haven’t used it myself, the reviews seem reasonable. Google ‘Facebook Meta WordPress Plugin’ for alternative options.